Danger: When Hydrogen Embrittlement Strikes

The best way to avoid hydrogen embrittlement is to use a material that is not susceptible. However, this usually means a lower-strength material, which may not always be acceptable, depending on the application. Another good way to avoid hydrogen embrittlement is to avoid exposure to hydrogen.

For example, if the hydrogen is being introduced by an electroplating operation, this might mean switching to mechanical plating, vacuum plating, or a dip-spin coating. It's also possible to reduce the risk of hydrogen embrittlement by "baking" the part as soon as possible after the operation that introduced the hydrogen (i.e. plating or welding). Baking means heating the part to a temperature that will allow the hydrogen to diffuse out. Typical baking temperatures range from 350°F to 400°F. For heat-treated parts, it's important to avoid temperatures that will reduce the part's strength. The longer the parts are in the furnace, the greater the likelihood that the hydrogen will be effectively removed. Some specifications require parts to be in the furnace for as long as 48 hours, while other specifications only require a minimum of three hours.

Hydrogen embrittlement is a delayed failure mode. It does not occur immediately, but strikes at random, usually within 24 hours to 48 hours after installation. The time to failure depends on the stress; the greater the stress a part is under, the more quickly failure will occur.

When I was a student, a professor told me about a problem with hydrogen embrittled fasteners that occurred during the construction of a major urban light rail system. Some of the fasteners were found to have failed within a day of installation. This led to an investigation that lasted several months. Eventually, the failures were attributed to hydrogen embrittlement. All of the remaining fasteners (several million dollars' worth) were therefore removed, scrapped out, and replaced with new fasteners. This was probably unnecessary; if the fasteners hadn't failed during the months of the investigation, they would most likely never fail. However, the city probably judged that, in the extremely unlikely event of a failure, the potential lawsuits could cost far more than the fasteners.

This is an interesting example of a failure mode that is not going to be easy to predict. It seems that, short of testing a sample of the parts after each process, one can only track the parts after production and remedy situations as they occur. This requires a detailed tracking of the products after delivery and detailed reports of problems. In general, quality control systems do this. By linking those databases with design data in a PLM system, another big theme of Design News lately, one can avoid the problem in the future.

@TJ McDermott: I would strongly recommend against derating hydrogen embrittled parts, if by derating you mean using the affected parts in a lower-stress application. The problem is that it's extremely difficult to predict what level of stress a hydrogen-embrittled part will fail at. Even residual stresses from the forming process may be enough to produce microcracks, which may propagate later in fatigue. If I knew that a part was likely to be hydrogen embrittled, I wouldn't recommend using it in any application.

The same line of argument applies to baking parts as a re-work method after a problem has been found. As I mentioned, it's important to bake parts as soon as possible after plating or welding. Some aerospace specifications require that this be done within one hour. The more time passes, the less effective baking will be. This is not so much due to the hydrogen being any more difficult to remove (although you will hear this claim), so much as the fact that microcracks may have already formed as a result of residual stress. Obviously, once cracks have formed, no amount of baking will heal them.

As naperlou pointed out, prevention of process-induced hydrogen embrittlement depends on having a good quality system in place. If you're plating or welding high-strength parts, you need to ensure that they are always baked at the proper temperature within the specified length of time.

Well, hydrogen embrittlement is a long known issue that has played havoc with metals at nuclear power plants and has lead to fuel rod cadding failures along with reduction of the rating of some containment vessels.

Derating is not a good option for dealing with hydrigen embrittlement due to the unpredictability of potnetial failure. The problem must be controlled/eliminted in process. I know of circumstance in which a critical flight control component fialed due to hydrogen embrittlement under strictly vibratory loading - no compression or tensile load at all. Reuslt was the death of a number of individuals.

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